1. Field of the Invention
The present invention relates to a spark plug for providing ignition in an internal combustion engine.
2. Description of the Related Art
A spark plug includes a tubular metallic shell and a rod-like insulator axially inserted into the metallic shell and has a spark discharge gap formed at one end of the insulator. The spark plug is mounted on an internal combustion engine by means of a male-threaded portion of the metallic shell such that the spark discharge gap is located within a combustion chamber of the engine. Since a combustion gas establishes high temperature and high pressure within the combustion chamber, sealing must be established against the outer surface of the insulator and against the inner surface of the metallic shell by a certain method in order to prevent leakage of the combustion gas. A conventionally known spark plug achieves such sealing by employing a sealing-material-powder layer formed from talc or the like. Specifically, a circumferential gap is formed between the inner circumferential surface of a rear end portion of the metallic shell and the outer circumferential surface of the insulator and is filled with a sealing material powder. While this sealing-material-powder layer is being compressed, a rear end portion of the metallic shell is crimped toward the insulator, thereby simultaneously performing assembly of the metallic shell and the insulator and sealing by means of the sealing-material-powder layer. Notably, when the insulator is subjected to an impact force, the sealing layer is compression-deformed to thereby alleviate the impact force; i.e., the sealing layer also serves as a cushion layer.
Recently, an increase in output of an internal combustion engine for use in an automobile or the like has been accompanied by an increase in the area occupied by an intake valve and an exhaust valve within a combustion chamber. Therefore, the size of a spark plug for igniting an air-fuel mixture must be reduced. With regard to a metallic shell, there is arising a demand for reduction in size with respect to a portion other than a male-threaded portion; specifically, a hexagonal portion (a tool engagement portion), which is located above the male-threaded portion and is adapted to be engaged with a wrench. This demand arises from the following reasons: employment of a direct ignition method—in which individual ignition coils are attached directly to upper portions of corresponding spark plugs—narrows an available space above a cylinder head; and the above-mentioned increase in area occupied by valves forces a reduction in the diameter of plug holes. As a result, the opposite side-to-side dimension of the hexagonal portion must be reduced to, for example, 14 mm or less from a conventionally available dimension of 16 mm or more.
The above-mentioned hexagonal portion is formed adjacent to a front side of a crimped portion of the metallic shell. The sealing-material-powder layer, which is compressed in the course of crimping, is formed in a section that overlaps the hexagonal portion with respect to the axial direction of the metallic shell. As the opposite side-to-side dimension of the hexagonal portion is reduced, the gap between the insulator and the metallic shell, which is to be filled with the sealing material powder, becomes narrower. As a matter of course, in order to increase the gap, the wall thickness of the hexagonal portion may be reduced, or the diameter of the insulator may be reduced. However, in the former case, since the wall of the hexagonal portion becomes excessively thin, the hexagonal portion is buckled so as to swell outward in the course of crimping. In the latter case, the insulator becomes too thin, resulting in insufficient strength and thus insufficient impact resistance. Therefore, when a hexagonal portion having a small opposite side-to-side dimension is to be employed, the gap to be filled with the sealing material powder is unavoidably narrowed.
In order to seal the insulator and the metallic shell against each other, the sealing-material-powder layer must be sufficiently compressed so as to assume a certain density or higher. In this case, before crimping is started, the sealing material powder must be charged into the above-mentioned gap while being subjected to preliminary compression effected by means of a punch or the like. However, in the case where a hexagonal portion having a small opposite side-to-side dimension is employed, as mentioned above, the gap between the insulator and the metallic shell becomes narrow; thus, uniform filling with the sealing material powder becomes difficult. Specifically, when a required amount of sealing material powder is charged into a deep, narrow gap at one time while being compressed by means of a punch or the like, friction acting on the outer circumferential surface of the insulator or on the inner circumferential surface of the metallic shell causes compression, to a biasedly large extent, of powder filling an upper portion of the gap located close to the punch, while an applied force is insufficiently transmitted to powder filling a lower portion of the gap. As a result, the upper portion of the gap is filled with the powder at high density, whereas the lower portion of the gap is filled with the powder at low density. Once such nonuniform density arises, subsequent compression effected by crimping merely compresses the powder filling the upper portion of the gap, thus failing to eliminate the nonuniform condition of filling, with a resultant impairment in gastightness or impact resistance.